An image sensor provides a grid of pixels, such as photosensitive diodes or photodiodes, reset transistors, source follower transistors, pinned layer photodiodes, and/or transfer transistors for recording an intensity or brightness of light. The pixel responds to the light by accumulating a charge—the more light, the higher the charge. The charge can then be used by another circuit so that a color and brightness can be used for a suitable application, such as a digital camera. Common types of pixel grids include a charge-coupled device (CCD) or complimentary metal oxide semiconductor (CMOS) image sensor (CIS).
Back-side illuminated sensors are used for sensing a volume of exposed light projected towards the backside surface of a substrate. The pixels are located on a front side of the substrate, and the substrate is thin enough so that light projected towards the backside of the substrate can reach the pixels. Back-side illuminated sensors provide a high fill factor and reduced destructive interference, as compared to front-side illuminated sensors.
A problem with back-side illuminated sensors is that in order to successfully process the backside of the wafer, a reticle pattern for a particular device layer must be correctly aligned to an existing pattern on the front side of the wafer. Alignment is the process of determining the position, orientation, and distortion of the patterns already on the wafer and then placing them in correct relation to the projected image from the reticle. Alignment marks are visible patterns placed on the reticle and the wafer to determine their position and orientation. However, alignment marks formed on the front side of the wafer are difficult to detect and align with conventional tools when patterning the backside of the wafer for processing. Improvements in alignment methods are desired to accurately and precisely pattern and process back-side illuminated sensors.